The purpose of a "Router" to is to route Internet traffic to the appropriate computer in a network. If there is no Internet involved in your network, then a Router is not the solution you are looking for.
At the moment, if all computers are using USB Wireless Network Adapters and connecting via an "Ad Hoc" network, you should probably be looking at replacing the Wireless Network Adapter in the MAIN PC with a "Wireless Access Point". This plugs into a conventional *wired* network adapter and, as its name suggests, provides an Access Point for wireless adapters to connect to a wired network (in this case just your main PC). You would then have an "Infrastructure" wireless network instead of an "Ad Hoc" one. The difference is that, instead of all three PCs talking directly to one another, they would all communicate via the Wireless Access Point, the main PC through its *wired* connection and the other two *wirelessly*.
The alternative is to set up a completely WIRED network. This would involve connecting each PC to a "Network Switch" by ethernet cable.